Exploring, drilling, completing, and operating hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses, added emphasis has been placed on information available for each stage of well completion and operation. From early seismic data to logging and profiling of the well to the continuous monitoring of well conditions throughout the life of the well, available well information plays a critical role in cost effective hydrocarbon recovery efforts.
In addition to the condition of the well itself, accurate information regarding downhole tools employed during well intervention is also of critical importance. For example, coiled tubing, wireline, and other downhole well applications involve the directing of downhole tools through the depths of the well. In many cases, the well may be of extreme depths, of a highly deviated nature, or involve other challenging characteristics making direct information regarding the downhole tool difficult to ascertain. For example, in a logging application, surface equipment at an oilfield may be used to deliver a logging head several thousand feet into a well by use of a wireline. Certain information regarding the wireline itself may be available through readings of the wireline taken at the surface. The amount of wireline disposed into the well or the tension imparted on the wireline may be measured at the surface. However, these readings may not constitute a true indication as to the depth to which the logging head has actually been disposed within the well nor the amount of tension imparted on the wireline downhole, at the location of the logging head. Such inaccurate or incomplete information may be adverse to the logging application. Inaccurate knowledge of downhole wireline tension at the logging head in particular may be catastrophic to the logging application. Unintentional breaking of the wireline at the logging head or a perceived inability to affect the depth or position of the logging head may result.
In order to help provide some level of accuracy regarding wireline tension at a downhole location of the logging head, a load cell is generally incorporated into a logging head or other tool at the end of the wireline. The load cell generally includes a strain gauge assembly configured to detect tension and electrically transmit tension readings uphole through the wireline. In this manner, direct readings of tension at the location of the logging head may be processed by uphole equipment and used in directing the logging application.
Unfortunately, the accuracy of the load cell is limited for reasons such as the downhole pressure of the well environment. That is, well pressure may increase significantly deeper and deeper within the well. However, certain load cells may remain isolated within the logging head, safeguarded from exposure to the harmful downhole environment. As a result, readings obtained by this type of load cell may fail to provide a true and accurate measurement of tension at the downhole location of the logging head within the well.
In order to address the inaccuracy of tension measurements provided by a load cell as indicated above, the load cell may instead be a ‘wet load cell’. That is, the load cell may be provided within a pressurizable, oil-filled chamber of the logging head. In such a situation, the pressurizable chamber within the logging head may be affected by movement of a piston in response to an influx of pressure at the other side of the piston. That is, the logging head may be configured to intentionally allow an influx of pressure at one side of a piston, whereas the other side of the piston may define the chamber housing the load cell. As such, the pressure within the chamber housing the load cell may be altered to roughly match the pressure of the outside well environment. Thus, in theory, a more reliable reading of tension at the location of the logging head may be obtained and transmitted uphole by the load cell.
Unfortunately, there remain significant limitations to the use of a wet load cell to provide downhole tension information as described above. Namely, while providing some degree of additional accuracy as compared to a load cell unaffected by downhole pressure, even a wet load cell has a significant degree of inaccuracy. For example, a wet load cell configuration may provide a degree of tension measurement accuracy to within no more than about +/−3%. While this may be sufficient for smaller load cells in wells of limited depths, a tension measurement error of 3% for a several thousand pound load cell in a deep well may amount to error in the neighborhood of several hundred pounds of load. Furthermore, the nature of the wet load cell involves exposure of strain gauges to oil as described above. As such, the useful life of the gauges and the load cell itself is generally in the neighborhood of about a year.